Mountain standing above the channels and cracks in the earth's crust are called volcanoes.
Often volcanoes similar to a cone-or dome-shaped mountain on top of which there is a crateror depression in the form of a funnel. Sometimes, as scientists say, the volcano "wakes up", and then there is its eruption. While on the surface facing the molten substance of the earth's crust and mantle of the Earth is called magma.
Eruption is a series of strong and weak explosions and outpourings of lava - a mixture of molten rock. The volume of the streamed lava can reach several tens of cubic kilometers. Eruptions are long, which can be observed for several years and even centuries, and briefly held for several hours. To their forerunners include the following hazards: earthquake, change of gas composition, sound (acoustic) drops and others.
Those volcanoes which from time to time thrown from his vent hot gases or steam are called valid. Also consider active volcano that recently erupted. On the Land of volcanoes around 500.
Many volcanoes behave quietly, without giving cause for concern. But one day the volcano can "Wake up", and then the consequences will be tragic. So it was with volcano arenas (elevation 4500 meters), located 150 kilometers from Bogota, Colombia's capital. The date of its last eruption was 1595. Since then, he was considered to be extinct, at its base, built the town of Armero. The tragedy occurred on 13 November 1985. Powerful bursts of melted snow and ice in the crater of the volcano and terrible streams of hot mud flowed on the town, whose inhabitants were sleeping in their homes. After the eruption there was only a dirt layer thickness of 5-6 meters. As a result of this eruption killed 20 thousand people.
There are volcanoes, mountains, and underwater volcanoes that are completely hidden under the water. "Waking up", these volcanoes spew not only magma, but whole water fountains.
There are mud volcanoes that spew streams of hot mud, and volcanoes-lake. The craters of volcanoes similar to a flat dish filled with boiling lava.